Vertical soi trench sonos cell

ABSTRACT

A semiconductor memory device in which a vertical trench semiconductor-oxide-nitride-oxide-semiconductor (SONOS) memory cell is created in a semiconductor-on-insulator (SOI) substrate is provided that allows for the integration of dense non-volatile random access memory (NVRAM) cells in SOI-based complementary metal oxide semiconductor (CMOS) technology. The trench is processed using conventional trench processing and it is processed near the beginning of the inventive method that allows for the fabrication of the memory cell to be fully separated from SOI logic processing.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/164,513, filed Nov. 28, 2005.

FIELD OF THE INVENTION

The present invention relates to a semiconductor memory device and amethod of fabricating the same. More particularly, the present inventionrelates to a semiconductor memory device in which a vertical trenchsemiconductor-oxide-nitride-oxide-semiconductor (SONOS) memory cell iscreated in a semiconductor-on-insulator (SOI) substrate thereby allowingfor the integration of dense non-volatile random access memory (NVRAM)cells in SOI-based complementary metal oxide semiconductor (CMOS)technology. The trench is fabricated using conventional trenchprocessing and it is formed near the beginning of the inventive methodthat allows for the fabrication of the memory cell to be fully separatedfrom SOI logic processing.

BACKGROUND OF THE INVENTION

Conventional flash memory products employing floating gate technologies,where the memory state is represented by the charge stored in aninsulated, but conducting layer between the control gate electrode andthe device channel region, typically use programming voltages (i.e.,writing and erasing) of 10V or higher. Because of the necessity ofproviding high voltage levels for floating gate memory operation,integration of floating gate memories with complementary metal oxidesemiconductor (CMOS) is problematic. First, present CMOS power supplies(V_(dd)) are approaching 1V. At these low supply voltages, it isdifficult to provide high voltage levels necessary for floating gateoperation—even with the use of charge-pumping circuits. Presently, oneis forced to provide a separate high voltage supply for floating gatememories, e.g., 3.3V, and the use of charge-pumping circuitry, inaddition to a lower voltage supply source for any CMOS logic associatedwith the chip. For mobile applications, the high supply voltagesnecessary for conventional floating gate memories severely limit thebattery life. Moreover, there is a severe cost penalty of integratingfloating gate memories with CMOS—it is estimated that as many as nine(9) additional lithography steps may be necessary.

Randomly-accessible semiconductor-oxide-nitride-oxide-semiconductor(SONOS) are also referred to as metal-oxide-nitride-oxide-semiconductor(MONOS); note that the terms SONOS and MONOS denote basically the sametype of memory cell except that polysilicon, is used as the SONOS gateconductor and a metal is used as the MONOS gate conductor) memory, isconsidered a viable replacement to floating gate memories due to themoderate operating voltages these memories employ, e.g., approximately5V. In such memory cells, the memory state is represented by the chargestored in an insulator layer located between the control gate electrodeand the device channel region. Depending on the programming conditionsof the memory cells, it may be possible, for example, to lower the highvoltage supply from 3.3V to 1.8V or even do away with the separate powersupply altogether, thereby prolonging battery life. In addition, sinceSONOS memories typically utilize uniform tunneling for programming,reliability problems usually associated with floating gate memories,such as hot hole injection, are avoided.

However, SONOS cells are not immune to scaling concerns. The moderatevoltages needed for programming put limits on the transistor design onecan employ. Short-channel effects, including punch-through, can easilyoccur even at these voltages, if the device channel lengths are tooshort. Increasing channel doping concentrations to deter punch-throughcan lead to lower junction breakdown and destruction of any memorycapability.

The scaling of SONOS memory cells has been limited to planar devicesthat are typically formed in bulk semiconductor substrates. The selectgate and memory gate are typically formed separately and the cell sizesof these types of cells are usually large. Recently, a SONOS planar bitcell with a 0.157 μm² cell size designed in 90 nm groundrules has beenreported. See, for example, C. T. Swift, G. L. Chindalore, K. Harber, T.S. Harp, A. Hoefler, C. M. Hong, P. A. Ingersoll, C. B. Li, E. J. Prinz,J. A. Yater; “An embedded 90 nm SONOS nonvolatile memory utilizing hotelectron programming and uniform tunnel erase,” IEDM Tech. Dig., pp.927-930, December 2002. Although smaller in size, the planar cell hasits limits as the channel length may be more difficult to scale in theforthcoming generations. In addition, the planar SONOS memory gate mustbe constructed separately from any CMOS gate process. Additional four(4) masks are required above and beyond the CMOS process to create theprior art SONOS cell array. Due to the necessity of separating the SONOSmemory gate from CMOS processing, integration with high-performance CMOScan become costly.

In view of the above, there is a need for providing a method tointegrate SONOS cells with SOI logic. Indeed, integration of planarSONOS cells on SOI is extremely difficult

-   -   the body of the planar devices are usually ‘floating’ and        channel lengths tend to be relatively short rather than long (to        avoid making fully depleted devices that are difficult to        control). These imposed constraints make the fabrication of        planar SONOS cells virtually impossible on SOI substrates.

Thus, it would be beneficial to NVRAM technology if there could be a wayof integrating SONOS on SOI where one can take advantage of utilizinghigh performance SOI logic devices together with the non-volatility ofthe SONOS memory cell.

SUMMARY OF THE INVENTION

The present invention provides a method wherein a vertical trench SONOScell is created in an SOI substrate thereby allowing for integration ofdense NVRAM cells in SOI-based CMOS technology. In accordance with themethod of the present invention, the trench in which the SONOS cell isfabricated is formed relatively early in the overall process flow. Thisallows the fabrication of the SONOS memory cell to be fully separatedfrom SOI logic processing. The method of the present invention iscapable of fabricating 4F² SONOS cells for highest density.

In broad terms, the method of the present invention includes the stepsof:

providing at least one vertical trench SONOS memory cell located withina semiconductor-on-insulator substrate that includes a buried insulatinglayer separating a top semiconductor layer from a bottom semiconductorlayer, said vertical trench SONOS memory cell includes a sourcediffusion located beneath said vertical trench and a select gate channellocated on exterior sidewalls of said vertical trench;

providing a Si-containing conductively filled recess within said buriedinsulating layer atop said vertical trench;

vertical outdiffusing dopant from said Si-containing conductively filledrecess to provide top and bottom outdiffused regions, said bottomoutdiffused region is in contact with said select gate channel; and

forming a silicided doped region in said top semiconductor layeradjacent to and in contact with said top outdiffused region.

In addition to the method described above, the present invention alsoprovides a non-volatile random access memory cell that comprises:

a semiconductor-on-insulator substrate comprising a top semiconductorlayer and a bottom semiconductor layer that are separated from eachother by a buried insulating layer; and

at least one vertical trench SONOS memory cell located within saidsemiconductor-on-insulator substrate, said at least one vertical trenchSONOS memory cell includes a source diffusion located beneath saidvertical trench, a select gate channel located on one side of saidvertical trench, an outdiffused/Si-containing bridge located above andin contact with said select gate channel, and a silicided doped regionlocated adjacent to and in contact with an upper portion of said bridge,wherein said bridge is present within said top semiconductor layer, saidburied insulating layer and said lower semiconductor layer.

A denser cell layout can be provided which allows for 1F spacing betweenisolated cells. The denser cell layout includes:

a semiconductor-on-insulator substrate comprising a top semiconductorlayer and a bottom semiconductor layer that are separated from eachother by a buried insulating layer;

at least one pair of vertical trench SONOS memory cells located withinsaid semiconductor-on-insulator substrate, each vertical trench SONOSmemory cell includes a source diffusion located beneath said verticaltrench, a select gate channel located on one side of said verticaltrench, an outdiffused/Si-containing bridge located above and in contactwith said select gate channel, and a silicided doped region locatedadjacent to and in contact with an upper portion of said bridge, whereinsaid bridge is present within said top semiconductor layer, said buriedinsulating layer and said lower semiconductor layer; and

a deep trench isolation region separating each pair of memory cells

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1J are pictorial representations (through cross sectionalviews) illustrating the basic processing steps employed in the presentinvention in fabricating a pair of vertical SOI trench SONOS cells ofthe present invention. Each of the cross sectional views is along thebitline (BL) direction, e.g., M2, which is through the pair of memorycells illustrated in these drawings.

FIG. 2 is a pictorial representation (through a top down view)illustrating a cell array layout including a plurality of SOI trenchSONOS cells, a pair of which is shown, for example, in FIG. 1J.

FIGS. 3A-3C are pictorial representations (through cross sectionalviews) illustrating the processing flow used in the present inventionfor fabricating a 4F² vertical SOI SONOS cell; these cross sectionalviews are also along the BL direction.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which describes a method for fabricating a trenchSONOS cell in an SOI substrate as well as the resultant NVRAM cellfabricated using the inventive method, will now be described in greaterdetail by referring to the following discussion and drawings thataccompany the present application. It is noted that the drawings of thepresent application are provided for illustrative purposes and, as such,the drawings are not drawn to scale.

It is observed that the method of the present invention to be describedin greater detail hereinbelow can provide a single trench SONOS cell ora plurality of SONOS cells, i.e., a memory cell array, can also beprovided. In the drawings depicting the inventive processing steps, apair of trench SONOS cells is illustrated.

The inventive method for fabricating a vertical trench SONOS cell is nowdescribed in reference to FIGS. 1A-1J. Specifically, FIG. 1A illustratesan initial structure 10 which is employed in the present invention forfabricating the vertical trench SONOS cell. The initial structure 10shown in FIG. 1A includes an array region of an SOI substrate 12 inwhich at least one of the inventive vertical trench SONOS cells will befabricated. Other regions of the SOI substrate, i.e., the SOI logicregion, lie to the periphery of the array region shown in this drawing.The structure 10 shown in FIG. 1A comprises thesemiconductor-on-insulator substrate 12 that includes a bottomsemiconductor layer 12A, a buried insulating layer 12B and a topsemiconductor layer 12C. The buried insulating layer 12B provideselectrical isolation between the top and bottom semiconductor layers 12Cand 12A, respectively. The initial structure 10 also includes a padstack 14 and an oxide hardmask 15 having a plurality of openings 16 ontop of the SOI substrate 12 as well as trench structures 18 that arelocated in the SOI substrate 12. It is noted that the SOI substrate 12in other regions can be protected with a blanket layer of the pad stack14.

With respect to the SOT substrate 12, the top and bottom semiconductorlayers (12C and 12A, respectively) thereof may comprise the same, ordifferent semiconductor material. Preferably, the top and bottomsemiconductor layers (12C and 12A, respectively) are comprised of thesame semiconductor material. The term “semiconductor material” denotesany material that has semiconducting properties. Illustrative examplesof semiconductor materials that can be used in the present inventioninclude, but are not limited to: Si, SiGe, SiC, SiGeC, Ge, GaAs, InAs,InP and other II/V or III/VI compound semiconductors. Layeredsemiconductor materials, such as, for example, Si/SiGe, are alsocontemplated herein. Typically, the semiconductor layers of the SOIsubstrate are comprised of a Si-containing semiconductor such as, Si,SiGe, SiC or SiGeC, with Si being even more typical.

The top and bottom semiconductor layers (12C and 12A, respectively) mayhave the same crystal orientation, or they may have different crystalorientations. In some embodiments, the top and bottom semiconductorlayers (12C and 12A, respectively) may be doped, undoped or containundoped and doped regions therein. The top and bottom semiconductorlayers (12C and 12A, respectively) may be strained, unstrained or theymay each include regions of strain and unstrain therein.

The buried insulating layer 12B is comprised of a crystalline ornon-crystalline oxide, nitride, or oxynitride. In a preferredembodiment, the buried insulating layer 12B is an oxide.

The SOI substrate 12 is fabricated using techniques that are well knownin the art including, for example, a layer transfer process, lamination,or by ion implantation and annealing. Typically, and when ionimplantation and annealing are used in forming the buried insulatinglayer 12B, a process known as SIMOX (Separation by Ion Implantation ofOxygen) is used.

The thickness of the various layers of the SOI substrate 12 may varydepending upon the technique that was used in forming the SOI substrate12. Typically, the top semiconductor layer 12C has a thickness fromabout 5 to about 200 nm, with a thickness from about 20 to about 100 nmbeing even more typical. The buried insulating layer 12B typically has athickness from about 5 to about 500 nm, with a thickness from about 50to about 200 nm being even more typical. The thickness of the bottomsemiconductor layer 12A is typically within ranges of a bulksemiconductor substrate.

The pad stack 14 is comprised of at least two material layers includinga bottom pad oxide layer and an upper nitride pad layer. The individuallayers of the pad stack 14 are not shown in the drawings of the presentinvention. The thickness of the pad stack 14 may vary depending on thenumber of material layers within the stack. Typically, the overallthickness for the pad stack 14 including a bottom pad oxide layer and atop pad nitride layer is from about 20 to about 500 nm. In an exemplaryembodiment, the pad oxide has a thickness from about 5 to about 10 nmand the pad nitride has a thickness from about 100 to about 200 nm.

The pad stack 14 is formed on the surface of the SOI 12, i.e., the topsemiconducting layer 12C, using one or more conventional blanketdeposition techniques such as, for example, chemical vapor deposition(CVD), plasma enhanced chemical vapor deposition (PECVD), evaporation,chemical solution deposition or atomic layer deposition. In addition todeposition processes, the various material layers of the pad stack 14can be formed by thermal oxidation or thermal nitridation. A combinationof the aforementioned techniques can also be used.

After forming the pad stack 14 on the SOI substrate 12, an oxidehardmask 15 is formed atop the pad stack 14. The oxide hardmask 15 isformed utilizing one of the techniques described above for forming thevarious material layers of the pad stack 14. The thickness of the oxidehardmask 15 may vary depending on the technique that was used to formthe same. A typical thickness for the oxide hardmask 15 is from about 20to about 400 nm.

A resist having trench openings 16 (the resist is not shown in FIG. 1A)in the array region is then formed by deposition of a resist materialand lithography. The lithographic step includes exposing the appliedresist to a desired pattern (i.e., a trench pattern) of radiation anddeveloping the exposed resist utilizing a conventional developer. Afterforming the resist having the trench openings 16, the pattern is firsttransferred to the oxide hardmask 15 utilizing an anisotropic etchingprocess such as reactive ion etching, ion beam etching, plasma etchingor laser ablation. The resist can be removed after the trench patternhas been transferred to the oxide hardmask 15, or it can remain on thestructure during the transfer of the pattern from the hardmask 15 to thepad stack 14 and then to the SOI substrate 12 and thereafter removed, asdescribed above. These later pattern transfer steps can be performedutilizing the same etching process as described above.

A plurality of trench structures 18 (two of which are shown in FIG. 1A),which are formed through the openings 16, having a depth, as measuredfrom the upper surface of the SOI substrate 12, from about 1 to about 2μm, are formed into the SOI substrate 12 in the array region. After thetrench structures 18 have been formed, and if not previously removed,the oxide hardmask 15 used to define the trench structures 18 is removedutilizing a conventional stripping process that selectively removesoxide, stopping on the nitride surface of the pad stack 14. In FIG. 1B,the structure does not include the oxide hardmask 15 since it is removedprior to performing the implantations, which will be described in detailhereinbelow.

Next, the cell's source diffusion 20 and select gate channel 22 can beformed by ion implantation processes. The structure after these twoimplants have been performed is shown in FIG. 1B. As shown, the sourcediffusion 20 is formed in the SOI substrate 12, e.g., the bottomsemiconductor layer 12A, about the exterior bottom wall of the trenchstructure 18, while the select gate channel 22 is formed into the SOIsubstrate 12 about the exterior sidewalls of each trench structure 18(also within the bottom semiconductor layer 12A).

The order of the implants used in forming the source diffusion 20 andthe select gate channel 22 is not critical to the present invention.Typically, however, the source diffusion implant occurs prior to theselect channel implant.

The source diffusion 20 may be doped with an n-type dopant atom (i.e.,an element from Group VB of the Periodic Table of Elements, such as, forexample, P, Sb or As) or a p-type dopant atom (such as an element fromGroup IIIB of the Periodic Table Elements, such as, for example, In, Bor BF₂). Typically, n-type dopants are used in forming the sourcediffusion 20. The implant energies used in forming the source diffusion20 is typically from about 10 to about 40 keV and ion doses from about1E15 to about 5E15 atoms/cm² are typically used to form a heavily dopedsource diffusion 20. Optionally, the sidewalls of the trench structure18 may be protected by a masking layer, such as a nitride spacer, duringthe implantation of the source diffusion 20. Since the dopantconcentration of the source diffusion 20 is much larger than that of theselect gate channel 22, the sidewalls may need to be protected to avoidcontamination of the channel by the source implant. The protectivespacers are removed prior to implantation of the select gate channel 22.

The select gate channel 22 has a doping polarity that is opposite tothat of the source diffusion 20. Typically, the selected gate channel 22is doped with a p-type dopant. The select gate channel implantation isperformed utilizing an angle implantation process in which a tilt anglefrom normal incident of about 5° to about 10° is typically employed. Thechannel doping required for the select gate channel 22 is dependent onthe thickness of the gate dielectric to be subsequently formed in thetrench structure 18 and the desired threshold voltage. An ion dose fromabout 1E13 to about 1E15 atoms/cm² is typically used for providing arange of threshold voltages (Vt) of about 0.3 to about 1.5V for a 10-15nm gate dielectric.

Alternatively, an array doping process could be used to set the Vt ofthe select gate device. Typically, in standard bulk logic technologies,a triple well is usually created to define P-wells. These wells consistof a deep implant (250 to 350 keV), a medium implant (of about 150 keV)and a shallow implant (of about 50 keV).

An oxide-nitride-oxide (ONO) dielectric stack 24 is then created withinthe trench structures 18, See, FIG. 1C. The ONO dielectric stack 24typically consists of a bottom tunneling oxide having a thickness fromabout 1.2 to about 3, preferably about 2, nm, a nitride layer having athickness from about 5 to about 15, preferably about 7.5, nm, and a topoxide having a thickness from about 2.5 to about 7.5, preferably about5, nm. The bottom tunneling oxide can be formed utilizing a conventionaldeposition process such as CVD or PECVD or a thermal oxidation processcan be used. The nitride layer of the ONO dielectric stack 24 istypically formed by a deposition process such, for example, PECVD or bya low pressure chemical vapor deposition (LPCVD) process. The top oxideis typically formed by a deposition process including, for example, CVD)or PECVD.

Next, a SONOS array block mask (not shown) is then formed which coversthe array portion of the SOI substrate 12 but leaves the trenchstructures 18 at the array edges (not shown) uncovered. The SONOS arrayblock mask may comprise a conventional photoresist material that isdeposited utilizing a conventional deposition process such as, forexample, CVD, PECVD, or spin-on coating, and patterned via lithography.A spacer reactive ion etching process is then performed to expose thebottom of the trench structures including source diffusion 20. Thisallows for the fabrication of contacts to the source diffusions 20 atthe array edges (not shown).

Referring back to the structure shown in FIG. 1C, a first Si-containingconductive material 26 such as, for example, polySi or polySiGe, isformed within each trench structure 18 atop the outermost layer of theONO dielectric stack 24. Preferably, the first Si-containing conductivematerial 26 is polySi that is doped with the same polarity as the sourcediffusion 20. The first Si-containing conductive material 26 serves asthe gate electrode of the memory cell, while the ONO dielectric stack 24acts as the gate dielectric of the memory cell. The first Si-containingconductive material 26 is formed utilizing a conventional depositionprocess. When doped polysilicon is used as the first Si-containingconductive material 26, it is preferred that an in-situ dopingdeposition process be employed. Other deposition process such as CVD,PECVD, sputtering, plating, and evaporation can also be used. Afterforming the first Si-containing conductive material 26, the firstSi-containing conductive material 26 is recessed below the buriedinsulating layer 12B of the SOI substrate 12. The recessing of the firstSi-containing conductive material 26 is performed utilizing ananisotropic etching process such as reactive ion etching.

A second Si-containing conductive material 28, which may comprise thesame or different, preferably the same, conductive material as the firstSi-containing conductive material 26, is then formed utilizing one ofthe deposition processes mentioned above. Typically, the first andsecond Si-containing conductive materials 26 and 28, respectively arecomprised of doped polySi. After deposition of the second Si-containingconductive material 28, the second Si-containing conductive material 28is then recessed approximately to the bottom surface of the buriedinsulating layer 12B.

Next, the ONO dielectric stack 24 is removed from vertical faces of thepad stack 14, the top semiconductor layer 12C and the buried insulatinglayer 12B providing the structure shown in FIG. 1C. The ONO dielectricstack 24 is removed from the aforementioned vertical faces utilizing aconventional stripping process well known to those skilled in the artwhich selectively removes the exposed portions of the ONO dielectricstack 24.

An undercut region (not specifically labeled) located beneath the topsemiconductor layer 12C is then formed laterally into the buriedinsulating layer 12B utilizing an etching process that selectivelyremoves buried insulating material. For example, and when the buriedinsulating material is comprised of an oxide, a HF etch is employed tocreate the lateral undercut region within the buried insulating layer12B. The lateral etch typically removes from about 50 to about 100 nm ofthe buried insulating layer 12B starting from the exposed vertical facethereof within the trench structure 18.

A third Si-containing conductive material 30, which may be the same ordifferent, preferably the same, conductive material as the first andsecond Si-containing conductive materials, is then formed. Typically,the third Si-containing conductive material 30 comprises doped polySi.The third Si-containing conductive material 30 fills the lateralundercut region provided in the buried insulating layer 12B mentionedabove. It is noted that it is possible that the third conducive material30 between adjacent trench structures 18 is merged rather than separatedas is shown in FIG. 1D.

The third Si-containing conductive material 30 is formed as describedabove and it is recessed to the bottom surface of the top semiconductorlayer 12C utilizing an etching process as described above as well.

A patterned resist 32 having an opening between adjacent trenchstructures 18 is then formed by deposition and lithography atop the padstack 14 providing the structure shown in FIG. 1D. Note that the padstack 14 between the adjacent trench structures 18 is exposed, i.e., notprotected by the patterned resist 32, and that some portions of thepatterned resist 32 may overlap the outer edges of the each of thetrench structures 18, as is shown, for example, in FIG. 1D.

The exposed portion of the pad stack 14 is then removed utilizing anetching process such as, for example, reactive ion etching. Thechemistry of the etching step is dependent upon the upper material ofthe pack stack 14. In the preferred embodiment of the present inventionin which the pad stack 14 includes an upper nitride layer, the uppernitride layer is first removed utilizing wet or dry isotropic etching.Typically hot phosphoric acid or HF/GL is used to remove a nitridelayer. The remaining pad material, e.g., oxide, within the opening isthen removed utilizing an etching process than selectively removes theremaining pad material. For example and when the remaining pad materialincludes an oxide, a fluorine containing etch can be used to remove theremaining oxide from within the opening. These steps expose the topsemiconductor layer 12C within the opening.

The patterned resist 32 is then removed utilizing a conventional resiststripping process well known in the art and the exposed topsemiconductor layer 12C and the third Si-containing conductive material30 are now removed within the opening. The removal of these materialswithin the opening may comprise a single reactive ion etching step,useful when the exposed top semiconductor layer 12C and the thirdSi-containing conductive material 30 within the opening are eachcomprised of a Si-containing material. When a single etch is used, a CF₄containing plasma is typically used. When the exposed top semiconductorlayer 12C within the opening is composed of non-Si-containing materials,a two step etching process can be used in which the first etchselectively removes the top semiconductor layer 12C within the opening,and the second etch selectively removes the third Si-containingconductive material within the opening. It is noted that this etchingstep or steps used at this point of the present invention utilize theremaining pad stack 14 as an etch mask so that etching only occurswithin the opening. Moreover, the target depth of this etching step orsteps is typically greater than 2200 Å and, as a result, adjacentregions of the third Si-containing conductive material 30, which mayhave been merged due to a large undercut etch of buried insulating layer12B, are now separated.

An oxidation process is then performed within the previously etchedregion forming an oxide liner 34 on exposed surfaces within thepreviously provided opening. The thickness of the oxide liner 34 formedat this point of the present invention may vary depending on thetechnique used in forming the same. Typically, the oxide liner 34 formedwithin the opening has an as deposited thickness from about 10 to about25 nm. It is noted that during this step of the present invention,dopant material within the third Si-containing conductive material 30outdiffuses into adjacent portions of the upper semiconductor layer 12Cas well as the lower semiconductor layer 12A providing outdiffusedregions 36. As is shown, outdiffused regions 36 in the lowersemiconductor layer 12A are in contact with an upper portion of thechannel gate 22. These outdiffused regions 36 in the lower semiconductorlayer 12A form the drain of the inventive device.

Next, a nitride liner 38, which serves as an etch stop layer, is formedon the oxide liner 34 and within the opening as well as atop the exposedsurface of the pad stack 14 that is located outside the opening. Thenitride liner 38 is formed utilizing a conventional deposition processsuch as, for example, CVD or PECVD. Typically, the nitride liner 38 hasa thickness from about 5 to about 25 nm, with a thickness from about 10to about 20 nm being even more typical.

The structure including the opening, the oxide liner 34, outdiffusedregions 36 and the nitride liner 38 is shown, for example, in FIG. 1E.

FIG. 1F shows the structure after filling the opening with an isolationtrench dielectric material, annealing and planarization, which stepsform a shallow trench isolation 40. This anneal step will also causeadditional outdiffusion in the structure in the regions indicated above.The isolation trench dielectric employed in the present inventionincludes an oxide such as TEOS or a high density plasma oxide. Althoughoxide trench dielectric materials are specifically described andillustrated, the trench dielectric material may also include a nitrideor oxynitride. Filling of the opening with a trench dielectric materialincludes the employment of any conventional deposition process such as,for example, CVD or PECVD. The anneal is typically performed at atemperature of about 850° C. or greater, with a temperature from about900° to about 1000° C. being more typical. The planarization processcomprises chemical mechanical polishing (CMP), grinding or a combinationthereof. In the structure shown in FIG. 1F, reference numeral 40 denotesthe shallow trench isolation region that includes the planarized trenchfill material. By “shallow” it is meant that the trench isolation regionhas a depth from about 100 to about 700 nm.

Next, a deglazing step which removes oxide from the structure isperformed. The deglazing step is performed utilizing conventionaltechniques well known in the art. This typically includes the use ofHF/ethylene glycol as a deglazing agent. After deglazing, the pad stack14 is removed from the structure utilizing a stripping process thatselectively removes the pad stack 14. When the pad stack 14 comprises anitride material, hot phosphoric acid can be used to selectively removethe nitride pad stack. In the event any divots form in the structureduring the stripping of the pad stack 14, the divots (not shown) can befilled with a dielectric material such as, for example, a nitride. Thedivot fill is performed utilizing a conventional deposition process suchas, for example, CVD or PECVD. When the pad stack 14 includes an oxideunder the nitride, the oxide pad stack is removed utilizing an etchingprocess that selectively removes oxide. For example, an HF containingetch can be used in removing the underlying pad oxide. Note that duringthese processing steps of the present invention, the shallow trenchisolation region 40 is thinned so that an upper surface thereof issubstantially coplanar with an upper surface of the top semiconductorlayer 12C.

The resultant structure after deglazing and removal of the entire padstack 14, including, for example, the nitride/oxide pad stack is shownin FIG. 1G. Note that an upper surface of the top semiconductor layer12C is now exposed. At this point of the inventive process, an arraywell (n- or p-) is formed utilizing a masked ion implantation process.The implantation is performed at an energy from about 150 to about 200keV and the dosage of implantation ions is typically from about 5E12 toabout 1E13 atoms/cm². The conditions providing above are exemplary andin no ways limit the scope of the present invention. It is noted thatthe array well is not shown in the drawings of the present application,but it would be formed within the lower semiconductor layer 12A aboutthe pair of trench structures shown.

Doped regions are now formed into exposed portions of the topsemiconductor layer 12C utilizing a conventional ion implantationprocess that is capable of forming doped regions within a semiconductorlayer. An annealing step may follow the ion implantation step. Theanneal activates and drives in the dopants that were previouslyimplanted. The doped regions may comprise an n-type dopant or a p-typedopant. It is noted that in the present invention, the doped regions,the outdiffusion regions 36, the third Si-containing conductive material30, the second Si-containing conductive material 28, the firstSi-conductive material 26 and the source diffusion 20 all have the sameconductivity (n or p-type). Preferably, each of the aforementionedregions and materials is of the n-conductivity type. Note that theselect gate channel 22 has the opposite conductivity than the regionsand materials mentioned previously in this paragraph.

After forming the doped regions, the doped regions are silicided (seeFIG. 1H, reference numeral 42). Silicidation includes applying a metalthat is capable of reacting with Si on the exposed surface of the topsemiconductor layer 12C. The metal used in the present invention informing a silicide can be composed of Ti, Ta, W, Co, Ni, Pt, Pd oralloys thereof. Typically, the metal includes one of Ti, Co, Ni, Pt oralloys thereof, with Ni or Pt alloys being particularly preferred in oneembodiment of the present invention. In some embodiments, a siliconlayer is applied to the surface of the top semiconductor layer 12C priorto metal formation. The metal is formed utilizing a conventionaldeposition process such as, for example, CVD, PECVD, plating, sputteringand other like deposition processes.

The metal can also include one or more alloying additives including, forexample, C, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Nb, Mo, Ru, Rh,Ag, In, Sn, Hf, Ta, W, Re, Ir or Pt, with the proviso that the one ofmore alloying additives is not the same as the metal used in forming thesilicide. When present, the one or more alloying additives is present inan amount from about 0.1 to about 50 atomic percent. The alloyingadditive can be added in-situ during the deposition of the metal, or itcan be introduced after the metal is deposited by ion implantation,plasma immersion or gas phase doping. The thickness of the as depositedmetal may vary depending upon the overall thickness of layer 12C.Typically, the thickness of the metal is from about 2 to about 20 nm,with a thickness from about 5 to about 10 nm being more typical.

In some embodiments, an oxygen diffusion barrier such as TiN or TaN isformed atop the metal prior to silicidation. The optional oxygendiffusion barrier, which is formed by a conventional deposition process,typically has a thickness from about 5 to about 50 nm.

The silicidation process includes a first anneal, removing any unreactedfirst and second metal from the structure together with the optionaloxygen diffusion barrier, and optionally a second anneal. The firstanneal is typically performed at lower temperatures than the secondannealing step. Typically, the first anneal, which may or may not form asilicide in its lowest resistance phase, is performed at a temperatureof about 300° C. or greater, with a temperature from about 350° to about650° C. being even more typical. The first anneal may be performed usinga continuous heating regime or various ramp and soak cycles can be used.The first anneal is typically carried out in a gas atmosphere such as,for example, He, Ar, N₂ or a forming gas anneal. The annealing time mayvary depending on the metals or metal alloys used in forming thesilicide. Typically, the annealing is performed for a time period fromabout 5 seconds to about 2 hours. The annealing process may be a furnaceanneal, a rapid thermal anneal, a laser anneal, a spike anneal or amicrowave anneal.

A selective wet etch process(es) can be used to remove any unreactedmetal as well as the optional oxygen diffusion barrier from thestructure. The second annealing step, if performed, is typically carriedout at a temperature of about 550° C. or greater, with a temperaturefrom about 600° to about 800° C. being more typical. The second annealmay be performed in the same or different gas atmosphere as the firstanneal.

In FIG. 1H, reference numeral 42 is used to denote the silicided dopedregions. FIG. 1H also shows the structure after forming a patternednitride barrier 44 and a patterned interlevel dielectric 46 thereon. Asshown, the patterned nitride barrier 44 and the patterned interleveldielectric 46 have openings therein. Some of the openings expose thesilicided doped regions 42. These openings are referred to as bitlinecontact vias and are labeled as reference numeral 48 in FIG. 1H. Theother openings which expose the second Si-containing conductive material28 are referred to as memory gate contact vias and are referenced byreference numeral 50 in FIG. 1H.

The structure shown in FIG. 1H is formed by first providing a blanketnitride barrier over the structure shown in FIG. 1G utilizing aconventional deposition process. The blanket nitride barrier typicallyhas a thickness from about 10 to about 25 nm. Next, a blanket interleveldielectric typically an oxide is formed over the blanket nitridebarrier. The blanket interlevel dielectric is formed by a conventionaldeposition process (such as CVD, PECVD or spin-on coating) and ittypically has a thickness from about 50 to about 300 nm. A photoresistmaterial (not shown) is then deposited by a conventional depositionprocess and it is subjected to lithography (including resist exposureand development) providing a patterned resist atop the blanket layers.The pattern in the resist is then transferred to the blanket layersutilizing one or more etching process. Typically, a reactive ion etchingprocess is used to form the openings shown in FIG. 1H. After etching,the patterned resist is stripped utilizing a conventional resiststripping process well known to those skilled in the art.

FIG. 1I shows the structure after filling the openings (48 and 50) witha conductive material and planarizing the same. The conductive materialcomprises polySi, polySiGe, a metal, a metal alloy, a metal silicide, ametal nitride or any combination thereof. Typically, the conductivematerial used in filling the openings is a metal such as, for example,Al, Cu, W, Pt, or alloys thereof, with W being highly preferred. Theconductive fill includes a conventional deposition process such as CVD,PECVD, plating, and sputtering, while planarization includes chemicalmechanical polishing (CMP), grinding or a combination thereof. In FIG.1I, the conductively filled bitline contact via 48 is referred to as thebitline contact 52, while the conductively filled memory gate contactvia 50 is referred to as the memory gate contact 54.

FIG. 1J shows the structure after forming additional interconnect levels56A and 56B and bitline (BL or M2) thereon. Each of the interconnectlevels comprises an interlevel dielectric 58 A and 58B includingconductive feature 60 located therein. The conductive features 60 andthe bitline (BL or M2) may comprise the same or different conductivematerial as the conductively filled vias mentioned above. The conductivefeatures atop the memory gate contacts 54 are labeled as M1 or MSG inthe drawing.

FIG. 2 is a top down view illustrating a memory array which includes aplurality of trench SONOS cells as depicted in FIG. 1J. In this drawing,cells A, B and C are specifically illustrated and are used in describingthe basic operation of the memory array. Electrical operations that maybe performed on individual memory cells in the memory array depicted inFIG. 2 will now be described. The basic programming operations of aSONOS cell are a) Write “0”, b) Write “1”/Erase “0, c) Read “1” or Read“0”. These operations will now be described by referring to FIG. 2 whichis a schematic showing a proposed SONOS cell array including Cells A-C.

Electrical operations that may be performed on each memory cell in anarray will now be described. The basic programming operations are a)Write “0”, b) Write “1”/Erase and c) Read “1” or “0”.

a) Write “0”: To write a “0” into Cell A, the bitline (BL) is biased at−5V, the memory select gate (MSG) is boosted to 5V and the array Pwellis biased to −5V. With these conditions, electrons are driven by thehigh field between the inversion layer of the memory cell and the memoryselect gate and they are trapped in the ONO of the memory cell portionof the trench. There is negligible current through the channels of theselect and memory transistors because the bitline and the deep trenchsource line are both at −5V. Inhibit cells sharing the same boosted MSG(cell C) will not be programmed since they see a low field between thebitline diffusion and memory select gate. Bitline current from cell C isrelatively small because the Pwell is biased at −5V. Additionally,inhibit cells sharing the same bitline (cell B), as the programmed cellwill not be written because their memory select gates are at −5V. Withelectron injection into the ONO, the threshold voltage of the memorydevice should increase considerably and should significantly reducecurrent when being read.

b) Write “1” (or Erase “0”): To write a “1” into cell A, the BL and thedeep trench source line are biased to +5V. The memory select gate isbiased to −5V, and the array Pwell is biased +5V. With these biasingconditions in the selected cells, hole injection into the ONO isdominant while trapped electrons are injected back into the bitlinediffusion of the memory device by the high field between memory gate andbitline diffusion. All cells sharing the same memory gate select line(e.g., Cell C, memory gate select line 1) undergo the write “1”/eraseoperation. Once all the same memory gate line are written “1”/erase,write “0” is performed selectively to produce the desired bitlinepattern. Inhibit cell B is not erased, nor does channel current flow,since its MSG, BL and deep trench source line are all at +5V.

c) Read: To read a “1 or “0” from cell A shown in FIG. 1I, the BL isbiased at a lower voltage (e.g., 1.5V) and the memory select gate isboosted to +1.5V. The array well and the deep trench source line aregrounded. If no current is detected, this means that the thresholdvoltage if the memory device is too high to turn on and, hence, a “0”has been written into the cell. If current is detected, this means thatthe threshold voltage of the memory device is low (and quite likelynegative) and the device is on; therefore, a “1” has been written intothe cell (i.e., a depletion mode planar memory device). Neighboringinhibit cells can be grounded or even biased slightly negative topresent the gate from being turned on.

In some embodiments of the present invention, a 4F² SOI SONOS cell iscreated that allows 1F spacing between adjacent isolated cells. Such acell and process for fabricating the same are depicted in FIGS. 3A-3C.It is noted that this embodiment of the present invention utilizes thesame basic processing steps used in forming the structure shown in FIG.1F. After providing the structure shown in FIG. 1F, an oxide hardmask 70is deposited on the structure utilizing a conventional depositionprocess. Typically, the oxide hardmask 70 is comprised of a dopedsilicate glass such as boron doped silicate glass. The oxide hardmask 70has a thickness from about 100 to about 300 nm. A patterned resist 72 isthen formed atop the oxide hardmask 70 utilizing a conventionaldeposition technique and lithography. The patterned resist 72 includesan opening 74 that is positioned above the shallow trench isolationregion 40 separating adjacent cells. The resultants structure includingthe oxide hardmask 70 and the patterned resist 72 is shown, for example,in FIG. 3A.

FIG. 3B shows the structure after etching through the opening 74 in thepatterned resist 72 stopping within the lower semiconductor layer 12A ata distance that is below that lower surface of the source region 20.This etching process includes reactive ion etching through the oxidehardmask 70 and the shallow trench isolation region 40; then etchingthrough liners 38 and 34, and finally reactive ion etching through lowersemiconductor layer 12A to a depth that is greater than the sourcediffusion 20. Typically, a reactive ion etching step is used in formingthe structure shown in FIG. 3B. In FIG. 3B, reference numeral 74′denotes the deep opening provided in the structure.

Next, and as shown in FIG. 3C, a flowable oxide such as a spin-on glassis formed within the deepen opening 74′ utilizing a conventionaldeposition process. After filling the deepen opening 74′ with theflowable oxide, the flowable oxide is planarized using the pad stack 14as an etch/polish stop layer. The planarized flowable oxide within thedeepen opening 74′ forms a deep trench isolation region 76 betweenadjacent memory cells. The processing as described in FIGS. 1G-1J is thepreformed as described above.

It is noted that the vertical SONOS memory cell depicted in either FIGS.1A-1J or FIGS. 3A-3C includes a semiconductor-on-insulator substrate 12comprising a top semiconductor layer 12C and a bottom semiconductorlayer 12A that are separated from each other by a buried insulatinglayer 12B and at least one vertical trench SONOS memory cell locatedwithin said semiconductor-on-insulator substrate. The at least onevertical trench SONOS memory cell includes a source diffusion 20 locatedbeneath a vertical trench 18, a select gate channel 22 located on oneside of said vertical trench, an outdiffused/Si-containing bridge(comprised of regions 36 and 30) located above and in contact with saidselect gate channel 22, and a silicided doped region 42 located adjacentto and in contact with an upper portion of said bridge, i.e., regions 36and 30. In accordance with the present invention, the ‘bridge’ presentwithin said top semiconductor layer 12C, said buried insulating layer12B and said lower semiconductor layer 12A.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A semiconductor structure comprising: a semiconductor-on-insulatorsubstrate comprising a top semiconductor layer and a bottomsemiconductor layer that are separated from each other by a buriedinsulating layer; and at least one vertical trench SONOS memory celllocated within said semiconductor-on-insulator substrate, said at leastone vertical trench SONOS memory cell includes a source diffusionlocated beneath said vertical trench, a select gate channel located onone side of said vertical trench, an outdiffused/Si-containing bridgelocated above and in contact with said select gate channel, and asilicided doped region located adjacent to and in contact with an upperportion of said bridge, wherein said bridge is present within said topsemiconductor layer, said buried insulating layer and said lowersemiconductor layer.
 2. The semiconductor structure of claim 1comprising a plurality of said vertical trench memory cells, whereineach pair of neighboring vertical trench memory cells are separated by ashallow trench isolation region.
 3. The semiconductor structure of claim1 comprising a plurality of said vertical trench memory cells, whereineach pair of neighboring vertical trench memory cells are separated by ashallow trench isolation region and a deep trench isolation region. 4.The semiconductor structure of claim 1 wherein said at least onevertical trench memory cell further comprises an oxide/nitride/oxidegate dielectric and a Si-containing gate electrode located within saidvertical trench, said gate dielectric and said Si-containing gateelectrode are located beneath said buried insulating layer.
 5. Thesemiconductor structure of claim 1 further comprising a bitline contactlocated above and in contact with said doped silicide region.
 6. Thesemiconductor structure of claim 1 further comprising a memory gatecontact located above and in contact with said at least one verticaltrench memory cell.
 7. The semiconductor structure of claim 5 furthercomprising a bitline located above and in contact with said bitlinecontact.
 8. The semiconductor structure of claim 6 further comprising aconductive feature embedded within an interlevel dielectric locatedabove and in contact with said memory gate contact.
 9. The semiconductorstructure of claim 1 wherein said source diffusion, said bridge and saidsilicided doped region all comprises an n-type dopant, and said selectgate channel comprises a p-type dopant.
 10. A semiconductor structurecomprising a semiconductor-on-insulator substrate comprising a topsemiconductor layer and a bottom semiconductor layer that are separatedfrom each other by a buried insulating layer; at least one pair ofvertical trench SONOS memory cells located within saidsemiconductor-on-insulator substrate, each vertical trench SONOS memorycell includes a source diffusion located beneath said vertical trench, aselect gate channel located on one side of said vertical trench, anoutdifussed/Si-containing bridge located above and in contact with saidselect gate channel, and a silicided doped region located adjacent toand in contact with an upper portion of said bridge, wherein said bridgeis present within said top semiconductor layer, said buried insulatinglayer and said lower semiconductor layer; and a deep trench isolationregion separating each pair of memory cells.
 11. The semiconductorstructure of claim 10 wherein said deep trench isolation region has adepth that is below that of said source diffusion of each memory cell.